1. Field of the Invention
The present invention relates to the structure of a battery module having a plurality of cells electrically connected, and a container for a battery module. Particularly, the present invention relates to the structure of a battery module improved in vibration resistance and heat resistance and a container thereof suitable as a battery module for electric vehicles having a large lithium secondary battery, for example, electrically connected.
2. Description of the Background Art
A battery module having a group of a plurality of unit cells electrically connected is conventionally known. Various types of battery modules with different voltages and capacities in addition to the supplemental function of displaying the remaining capacity and the like are produced in order to correspond to various applications, and are now available on the market as a portable power supply.
The unit cell forming the battery module has a structure in which a collector piece is welded to the current collector. Upon exertion of vibration, the collector piece fixed by welding will receive stress to result in fraction of the collector piece itself or the weld portion thereof.
Electric vehicles are recently known as the new application for a battery module. Since the battery module for an electric vehicle is generally used in the situation where vibration is constantly exerted, there is the need of improving the poor vibration resistance arising from the internal structure of the unit cell. There is also the heat radiation demand for a structure that discharges outside the heat generated from the unit cell in order to prevent increase in the temperature of the unit cell that becomes the cause of degrading the unit cell performance.
Japanese Patent Laying-Open No. 6-203823, for example, discloses a battery developed to accommodate the aforementioned need of improving the vibration resistant property. According to the technique disclosed in this publication, the structure of the interior of the unit cell is modified. The power generation member is held elastically within the unit cell by connecting the current collector and the collector piece by means of compression. The problem of stress being exerted on the collector piece that is fixed by welding when vibration occurs to result in fracture is solved. However, this method by compression connection requires the component and the procedure of fabricating a unit cell to be partially modified since modification of the internal structure of the unit cell is indispensable.
Although vibration resistance is improved in the unit cell according to the compression connection method, resistance at the connecting section increases since the connection between the collector piece and the current collector is not electrically sufficient in comparison to the connection by the conventional welding method. As a result, the voltage drop is great when a great current is discharged. Problems will be encountered in the battery characteristics such as reduction in the cell capacity. Thus, there was a problem that a battery module using unit cells according to such a method is degraded in performance than a battery module using conventional cells.
As another approach of accommodating the demand for vibration resistance, a structure of buffering the vibration by securing the unit cell in a container using rubber is generally known. Although vibration resistance is improved by the secure structure using rubber, degradation in the performance of the unit cell caused by the heat generated from the charge/discharge of the unit cells in the battery module cannot be suppressed since the heat dissipation ability of rubber is low.
Therefore, when improvement in vibration resistance is required in a battery module of the conventional type, the unit cell per se had to be modified. Furthermore, when a great current is to be output from this cell, the voltage drop will increase to reduce the capacity of the unit cell since the internal resistance is high. There was a problem that a battery module using such unit cells has low performance.
As a technique of improving the heat radiation property of a battery module, Japanese Patent Laying-Open No. 9-86188 discloses the art of cooling a unit cell that generates heat in response to the charge/discharge by providing a hole in the container for cooling.
However, for a battery module formed of a plurality of unit cells according to such a conventional structure, the difference in temperature between the outer side unit cell located outside in the proximity of the hole for cooling and the inner side unit cell located further away from the hole could not be suppressed sufficiently. Difference in temperature between the unit cells in a battery module will result in different conditions for the charge and discharge in respective unit cells. The capacity of each of the unit cells in the battery module will differ to degrade the essential performance of the battery module.
In view of the foregoing, an object of the present invention is to provide a high performance battery module that allows improvement of vibration resistance and that suppresses degradation of the performance caused by the heat generated from the unit cell without modifying the internal structure of a conventional unit cell.
Another object of the present invention is to provide a battery module suppressed in the adverse effect caused by variation in the temperature of the unit cells within the battery module.
According to an aspect of the present invention, a battery module includes a plurality of electrically connected unit cells fixed in a container, and a buffer member at a void between the inner wall of the container and the unit cell.
According to the battery module of the above structure, the requirement of the vibration resistant aspect can be met by the buffer member in the battery module being a heat conductive elastic body while the heat generated from the unit cell can be easily discharged via the buffer member. Therefore, a battery module suppressed in degradation of the performance of the unit cell caused by heat can be provided. In accommodating the needs of both vibration resistance and heat dissipation, the internal structure of the unit cell does not necessarily have to be modified. There is an advantage that the component and procedure of fabricating a unit cell do not have to be modified.
In the battery module of the present invention, the buffer member preferably has metal particles mixed in rubber. Accordingly, the shape of the buffer member can be selected arbitrarily. Therefore, a battery module that can easily be applied to the currently available battery module and suppressed in modification cost can be provided.
By using metal particles including aluminum or copper to be mixed into the rubber forming the buffer member, the heat generated at the unit cell can be conveyed to the container efficiently. Therefore, reduction in the performance caused by the heat of the unit cell can be suppressed. Since these materials are relatively economical and easily available, the cost of the buffer member can be lowered.
The material of the rubber forming the buffer member preferably includes one or more of the material of styrene rubber, butadiene rubber, styrene butadiene rubber and urethane rubber. These materials are advantageous in that aggregation of the metal particles does not easily occur since the materials are miscible with the metal particles in addition to having the property sufficient for a buffer member. By mixing metal particles into these rubber materials, a buffer member of uniform property can be obtained. Thus, a battery module stable in quality can be provided.
The thickness of the buffer member is preferably 2-15 mm. By selecting such a range of thickness for the buffer member, a buffer member that has sufficient heat dissipation effect with small volume can be obtained. If the thickness is less than 2 mm, the property as a buffer member is not sufficient. If the thickness is greater than 15 mm, the effect of heat dissipation is reduced, and the energy density for the battery module will degraded. By virtue of the present invention, a battery module of high energy density using a buffer member of small volume can be provided.
According to an aspect of the present invention, a container of the battery module of the present invention accommodates a plurality of unit cells in an electrically connected state. A buffer member is provided between the inner wall of the container and the stored unit cell. The buffer member is formed of a heat conductive member.
According to the battery module container of the above structure, the heat generated by the unit cell can be discharged efficiently through the buffer member in the state where the unit cells are stored in the container.
According to another aspect of the present invention, a battery module includes a plurality of unit cells, a container in which the unit cells are stored, and a plurality of cell secure plates having a void formed into which the unit cell is inserted to fixedly hold the unit cell inserted in the void within the container. The cell secure plate is made of metal.
When the cell generates/absorbs heat by the charge/discharge in the battery module of the above structure, variation of cooling for the unit cells is suppressed by the heat conductivity of the cell secure plate formed of metal. The difference in temperature occurring between the unit cells can be reduced. Thus, a high performance battery module suppressed in difference between the property of respective unit cells can be provided.
In the battery module, the cell secure plates are installed at an interval of not more than ⅓ the length of the unit cell. By setting such an installation distance between the cell secure plates, a battery module suppressed in variation in temperature of respective unit cells can be provided.
In the battery module, the cell secure plate and the unit cell are preferably connected with a heat conductive resin. Such a connection allows the heat generated at the cell to be reliably conveyed to the cell secure plate. Since the cell secure plate and the outer side of the cell are in close contact, each cell can be reliably fixed.
In the battery module, the cell secure plate is formed of a punching metal having a porosity of 30-70%. By using such a material for the cell secure plate, the weight of the battery module can be reduced without degrading the strength. A battery module of high energy density per unit weight can be provided.
From the standpoint of improving the cooling efficiency, the cell secure plate is installed in a slanted manner with respect to the plane perpendicular to the longitudinal direction of the unit cell.
According to a further aspect of the present invention, a container for the battery module of the present invention has a void formed in which a plurality of unit cells are inserted to store the unit cells in an electrically connected state. A plurality of metal cell secure plates are provided to fixedly hold the unit cells within the container of the battery module with the unit cells inserted in the void.
According to the container for the battery module of the above structure, generation of difference in the temperature between the unit cells can be suppressed by the heat conductivity through the metal cell secure plate in the state where the unit cells are stored in the container. Since variation in the property of the unit cells can be suppressed, the performance of the battery module can be improved. The cell secure plate formed of metal is not easily deformed with respect to external stress. Therefore, the mechanical strength of the battery module can be improved.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.